Interpretive Summary: The Asian H5N1 avian influenza (AI) viruses have changed from producing no disease or mild respiratory infections in ducks to some strains causing severe disease and death. Differences in the disease induced in ducks by four of these viruses as well as the effect of the age of the ducks on the outcome of infection were studied. Three of the viruses induced high mortality in 2-week-old ducks and severe neurological signs. Neurological signs were also observed in 5-week-old ducks infected with one of these viruses; however mortality was low. The brain, heart and pancreas were the organs most commonly affected and the virus was most often detected by immunohistochemistry in these organs. The fourth virus studied did not induce neurological signs in 2-week-old-ducks, but did produce moderate mortality. This virus caused no clinical signs or death in 5-week-old ducks. All viruses studied were isolated from internal organs, demonstrating systemic infection. Genetic analysis of the viruses studied showed changes in several genes, but none clearly associated with pathogenicity. In conclusion, the pathogenicity of circulating H5N1 AI viruses in ducks varies depending on the virus strain and the age of the duck and correlates with the level of viral replication in tissues.

Technical Abstract:
The Asian H5N1 avian influenza (AI) viruses have changed from producing no disease or mild respiratory infections in ducks to some strains causing systemic disease and death. Differences in pathogenicity between four of these viruses as well as the effect of host age on the outcome of infection were studied in ducks. Three of the viruses were highly lethal in 2-week-old ducks and induced severe neurological dysfunction. Neurological signs were also observed in 5-week-old ducks inoculated with one of these viruses; however mortality was low. The brain, heart and pancreas were the organs most consistently affected and viral antigen was most often detected by immunohistochemistry in the parenchyma of these organs. The fourth virus studied did not induce neurological signs in 2-week-old-ducks, but did produce moderate mortality. This virus caused no clinical signs or death in 5-week-old ducks. All viruses studied were isolated from oropharyngeal and cloacal swabs, and also from brain, heart, lung and muscle tissues, demonstrating systemic infection. All viruses evaluated transmitted efficiently to contact ducks. Phylogenetic analysis of the viruses studied and other Asian H5N1 AI viruses with diverse pathogenicity in ducks, showed changes in several genes, but none clearly associated with pathogenicity. In conclusion, the pathogenicity of circulating H5N1 AI viruses in ducks varies depending on the virus strain and the age of the duck and correlates with the level of viral replication in tissues. High titers of virus in organs, high viral shedding, and variable mortality, enable ducks to circulate H5N1 HPAI viruses.